Wavelength division multiplexing (WDM) is being introduced as a means of increasing the capacity of optical fiber transmission systems. In a WDM system, each individual fiber carries a number of optical signals having different wavelengths. When these optical signals are transmitted over long distances, periodic regeneration of the optical signals is necessary. Currently, this amplification is effected either by demultiplexing the different wavelengths and then converting the optical signals to corresponding electrical signals which are regenerated and then reconverted to optical signals or by using optical amplifiers, e.g. Erbium Doped Fiber Amplifiers (EDFA). Optical amplifiers do have the advantage of both relatively low cost and the ability to amplify all used wavelengths without the need for demultiplexing and optoelectronic regeneration.
WDM systems currently under development will have thirty or more channels, i.e., modulated optical signals with different wavelengths (known as Dense Wavelength Division Multiplexing, DWDM). These DWDM systems are demanding for optical amplifiers which, especially considering the cascadation of a plurality of optical amplifiers along the transmission path of the DWDM system, have only very limited tolerances in certain parameters. Among these parameters gain flatness and gain tilt are of special importance. Problems with gain tilt may arise from aging of the DWDM system, from temperature effects, from different attenuation slopes of fiber used to form the transmission path or from stimulated Raman scattering.
It is known that gain tilt and gain flatness of the optical amplifiers can be optimized by controlling the input power of the optical amplifier. In European Patent Application EP 0 637 148 A1, a WDM system is described wherein transmitters are used which have means for associating an identification signal with each transmitted wavelength and wherein each optical amplifier of the transmission path has means for determining from the identification signals the number of wavelengths present on the transmission path whereby to control the power of the different channels. The use of identification signals for each transmitted wavelength also allows for maintaining a power balance between different wavelength channels in order to maintain the necessary gain flatness. This is achieved by determination of the amplitudes of individual identification signals associated with the transmitted wavelengths.
The known WDM system has the disadvantage of associating an identification signal with each transmitted wavelength channel. In addition, it has the disadvantage that in WDM systems it is not guaranteed that every wavelength channel is present all the time. This causes problems if the amplitude of the individual identification signal normally associated with a missing wavelength channel is used to maintain gain flatness as described above.